1. Field of the Invention
Embodiments discussed herein relate to a semiconductor module and a method for manufacturing the same. In particular, they relate to a semiconductor module used in applications including an inverter device, such as a motor drive device, and a power conversion device, such as a switching power supply device, and to the method for manufacturing the same.
2. Background of the Related Art
In a semiconductor module for power conversion, a plurality of semiconductor chips for power, such as power transistors or diodes for power conversion, is integrated into one package. In such a semiconductor module, circuit wiring suitable for a desired application is performed in advance inside a package, thereby contributing to miniaturization of the whole application device. In the semiconductor module for power conversion, a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) or an IGBT (Insulated-Gate Bipolar Transistor) is generally used as a power transistor.
An element for power conversion is configured so as to be exposed to the outside of a semiconductor module via a copper foil surface on an insulating substrate and via the insulating substrate, in order to reduce by heat radiation the heat generated by the power loss of the element. Here, the heat radiation is performed from the exposed surface using a radiator (e.g., see Japanese Laid-open Patent Publication No. 2013-258321 (Paragraph [0043], FIG. 2)). That is, in this Japanese Laid-open Patent Publication No. 2013-258321, an insulating layer is arranged in the surface of a thermally-conductive metal plate, a main circuit wiring pattern is formed on the insulating layer, and a semiconductor chip is bonded onto the main circuit wiring pattern. The heat generated by this semiconductor chip is transferred to the metal plate via the main circuit wiring pattern and the insulating layer, and is radiated by a heat sink bonded to this metal plate.
Moreover, in the element for power conversion, a desired circuit is formed from a lead frame, a conductive wire, and/or a wiring pattern formed on a substrate, and the circuit is directly connected or indirectly connected via a wire or the like, to a terminal which is the means for electrically connecting to the outside.
Furthermore, a module called IPM (Intelligent Power Module) which is a semiconductor module having a control IC (integrated circuit) mounted thereon has been commercialized and widely used. The control IC has a driver function to drive the element for power conversion and a function to detect abnormal states of the element, such as overcurrent, and protect the element. Since the control IC generates a small amount of heat, and in order to avoid the noise of the element for power conversion, the control IC is mounted at a position away from the substrate on which the element for power conversion is mounted. In Japanese Laid-open Patent Publication No. 2013-258321, the control IC is mounted on a wiring pattern formed inside a case.
Moreover, in the IPM, there is also a module having mounted therein a current detection element, a temperature detection element, a snubber element, and/or passive components, such as a capacitor connected to supply a stable electric power to the control IC. The semiconductor module having these elements mounted therein and having a desired electric connection made therein is resin-sealed to complete the IPM.
For example, in the IPM disclosed in Japanese Laid-open Patent Publication No. 2013-258321, elements for power conversion, such as an IGBT and an FWD (Free Wheeling Diode), are mounted, by soldering, onto an insulating substrate in which a wiring pattern is formed in advance. In a terminal case holding this insulating substrate, a lead frame and a PPS resin (Polyphenylene Sulfide Resin) are integrated by insert molding. The terminal case is molded by injecting the PPS resin into a mold which is fixed and set by a fixing pin that is placed so that the lead frame may neither displace nor deform due to a resin pressure during molding, or a fixing pin such as an ejector pin, but an opening portion formed at that time (hole by the fixing pin) is sealed with the resin by moving the fixing pin during molding. In the terminal case, elements, such as a control IC, are mounted on a lead terminal with an adhesion means, such as an Ag (gold) paste. Subsequently, the insulating substrate is bonded to the terminal case with an adhesive, electrical connection (ultrasonic bonding) is made using an aluminum wire so as to form a desired circuit across the element, the wiring pattern of the insulating substrate, and the lead frame, and then the terminal case is sealed with a cast resin so as to constitute a semiconductor module.
It is known that a bonding defect may be caused in ultrasonically bonding a bonding wire to a lead terminal (e.g., see Japanese Laid-open Patent Publication No. 2004-134518 (Paragraphs [0048]-[0056], FIGS. 4 to 7)). According to this Japanese Laid-open Patent Publication No. 2004-134518, in manufacturing a terminal case, due to a difference in the linear expansion coefficient between a resin and a metal lead terminal, a lead terminal closely attached to the terminal case may depart from the case when a high-temperature resin gets cold. Moreover, because there is no adhesive force between the lead terminal and the PPS resin, a very small gap will be produced between the both even if the both are formed so as to be closely attached to each other. In a state where a lead terminal is floating from the case, in attempting to ultrasonically bond a bonding wire to the lead terminal, the ultrasonic vibration energy will be absorbed by the lead terminal, which causes a bonding defect.
In contrast, in Japanese Laid-open Patent Publication No. 2004-134518, a through-hole is formed in a case directly under an ultrasonic bonding portion of a terminal plate serving as a lead terminal, and a support member insertion port is formed, also in a heat sink bonded to the case, at the position corresponding to the through-hole. In ultrasonically bonding a bonding wire, the bonding wire is ultrasonically bonded to the upper surface of the terminal plate using a rod-like tool while the lower surface of the ultrasonic bonding portion of the terminal plate is supported with the fixing pins arranged in the support member insertion port and the through-hole. Note that, because the terminal plate faces the through-hole, the lower surface of the terminal plate is exposed to the air to improve the radiation performance.
Moreover, in Japanese Laid-open Patent Publication No. 2004-134518, in the lower surface of a frame-shaped case made of an insulating resin and having an opening formed in the center portion thereof, a plate-like heat sink formed from a thermally conductive metal is arranged so as to plug the opening with the heat sink. Although an adhesive is used in bonding such a resin and metal, if the adhesive is excessively applied more than needed, then a high bonding strength may be obtained but the adhesive may protrude from the bonding area. The adhesive flowing out to the outside of the case will deteriorate the appearance of the case, and furthermore the adhesive flowing into the case will contaminate the bonding portion formed by ultrasonic bonding and reduce the bonding strength.
As a method for suppressing such protrusion of the adhesive, there is known a method in which a plurality of grooves is formed in the surface facing the heat sink of a case, among the plurality of grooves, a groove in the vicinity of the center is used as the adhesive application groove, and the outer and inner grooves are used as the adhesive outflow prevention grooves (e.g., see Japanese Laid-open Patent Publication No. 2012-15349 (FIG. 1)). Thus, even if the adhesive protrudes from the adhesive application groove, the adhesive is accommodated into the adhesive outflow prevention groove, thus preventing the adhesive from flowing out beyond the adhesive outflow prevention groove.
The conventional semiconductor module has a problem that the equipment cost increases because when a bonding wire is ultrasonically bonded to a lead frame, a pin-like pressing jig is needed for pressing the face opposite to an ultrasonic bonding face and in addition the pressing jig is complicated. Moreover, there is also a problem that the manufacturing throughput decreases because the pressing jig needs to be attached and detached in ultrasonic bonding and thus the attachment and detachment take a time.